Electronic devices must meet maximum electromagnetic interference (EMI) radiation limits as specified by the United States Federal Communications Commission (FCC) and other comparable regulatory agencies in other countries. New FCC requirements call for personal computer (PC) motherboards to be able to pass electromagnetic interface (EMI) tests “open box,” so manufacturers will not be able to rely on the shielding provided by the case in meeting EMI requirements.
An EMI suppression-enabled clock integrated circuit (IC) can reduce the system radiated EMI. The reduction in radiated EMI can result in dramatic cost savings for the system. Conventional techniques for reducing EMI include ground planes, filtering components, shielding, and spread spectrum modulated system clocks.
In the spread spectrum technique, instead of concentrating all of a frequency reference's energy on a single frequency, the energy is spread out by modulating the frequency. The modulation results in the energy being spread over a frequency range, instead of being concentrated on one particular frequency. Since the FCC and other regulatory bodies are concerned with peak emissions, not average emissions, the reduction in peak energy due to spread spectrum modulation will help a product meet FCC requirements.
Referring to FIG. 1, a block diagram of a circuit 10 illustrating a conventional phase lock loop based spread spectrum clock generator is shown. The circuit 10 generates a signal OUT in response to a reference signal REF. The circuit 10 comprises a phase detector 12, a charge pump 14, a low pass filter 16, a linear voltage controlled oscillator (VCO) 18, a feedback divider 20 and a spread spectrum circuitry block 22. The phase detector 12 has a first input that receives a reference signal REF and a second input that receives a feedback signal FEEDBACK. An output of the phase detector 12 presents a pump signal PUMP to an input of the charge pump 14. The charge pump 14 generates a control signal VIN in response to the signal PUMP. The signal VIN is filtered by a low pass filter 16 and presented to an input of the linear VCO 18. The linear VCO 18 generates a signal OUT in response to the signal VIN. The signal OUT has a frequency that is linearly dependant upon a voltage level of the signal VIN. The signal OUT is presented to an input of the feedback divider 20. The feedback divider 20 generates the signal FEEDBACK in response the signal OUT and a control signal SSM. The signal FEEDBACK is presented to an input of the spread spectrum circuitry block 22. The spread spectrum circuitry block 22 generates the signal SSM in response to the signal FEEDBACK and a set of ROM codes 24. The frequency of the signal OUT is modulated in response to the signal SSM.
Referring to FIG. 2, a line graph illustrating the frequency of the signal OUT versus the voltage level of the signal VIN is shown. The frequency of the signal OUT is linearly related to the voltage level of the signal VIN. Changes in the frequency of the signal OUT are related to changes in the voltage level of the signal VIN by a constant value.
The set of ROM codes 24 of a conventional digital spread spectrum clock generator are optimized for a particular frequency. For good performance, a conventional spread spectrum clock generator requires a separate set of ROM codes for each frequency at which the spread spectrum clock generator will operate. In order to get the best EMI reduction, every applied frequency needs a unique set of ROM codes. A full range of applied frequency can vary from 50 MHz to 170 MHz. In order to cover the full range using conventional spread spectrum clock generating devices, at least 5 devices are required. Each of the devices is configured to operate over a 10 MHz portion of the full range. However, the best performance is at the optimized frequency of the ROM codes of each device. The performance of the devices at other than optimized frequencies will be compromised.
A set of spread spectrum ROM codes occupies a large portion of a spread spectrum device. Multiple sets of ROM codes requires large amounts of space. Without multiple sets of ROM codes, the effectiveness of spread spectrum modulation is compromised as frequency changes.
Referring to FIGS. 3a–3d, oscilloscope traces illustrating degradation of the spread spectrum modulation of the signal OUT at various frequencies. As the frequency of the signal OUT changes, modulation of the frequency of the signal OUT varies from an ideal profile (i.e., FIG. 3b). FIG. 3b illustrates the signal OUT having a mean frequency of 79 MHz. When the signal OUT has a mean frequency of 79 MHz, modulation of the signal FOUT follows an ideal profile for spread spectrum modulation. FIG. 3a illustrates changes in the modulation profile when the signal OUT has a mean frequency of 48 MHz. FIG. 3c illustrates the signal OUT having a mean frequency of 107 MHz. When the signal OUT has a mean frequency of 107 MHz, modulation of the signal OUT is similar to a triangle waveform. Referring to FIG. 3d, as the mean frequency of the signal OUT increases to 149 MHz, modulation of the signal OUT becomes sinusoidal.
Another conventional approach that uses a single set of ROM codes for multiple frequencies is to adjust the bandwidth of the spread spectrum clock generator using an external low pass filter. The components of the external filter must be changed to compensate for each change in frequency. However, the external low pass filter provides very limited improvement.
A spread spectrum clock generator that uses a single set of ROM codes, requires no adjustment, and could generate any frequency in a wide range of frequencies, would be desirable.